In recent years, research and development have been extensively conducted on thin film transistors and semiconductor devices using thin film transistors.
Silicon is most widely used as a semiconductor material of thin film transistors. As semiconductor devices, display devices (liquid crystal display devices, EL display devices, and the like) are given, for example.
In thin film transistors applied to display devices, amorphous silicon and crystalline silicon are used in accordance with the usage. For example, amorphous silicon is used for large display devices. However, in the case where higher field effect mobility is desired to be obtained, for example, in the case where a liquid crystal display device is operated with double-frame rate driving, amorphous silicon does not sufficiently meet such a need.
Thus, when low-temperature polysilicon which is a kind of crystalline silicon is used for a liquid crystal display device, a driver circuit can be formed over a glass substrate, and a narrower frame and a high definition of the display device can be obtained. Alternatively, when high-temperature polysilicon which is a kind of crystalline silicon is used, higher definition can be obtained as compared to the case of using low-temperature polysilicon, so that high-temperature polysilicon is applied to pixels in a projector, or the like.
However, a laser crystallization process is included in the manufacture of low-temperature polysilicon in general, and thus, low-temperature polysilicon cannot be used for a large substrate. Since an expensive quartz substrate is used for the manufacture of high-temperature polysilicon, a technique of manufacturing thin film transistors having sufficiently high field effect mobility over an inexpensive glass substrate with a large area has not been established yet.
Thus, as one technique of manufacturing thin film transistors having sufficiently high field effect mobility over an inexpensive glass substrate, thin film transistors formed using microcrystalline silicon have been developed. Further, in recent years, thin film transistors formed using an oxide semiconductor have been developed (e.g., Patent Document 1 and Patent document 2). The use of an oxide semiconductor allows thin film transistors to obtain sufficient mobility, so that thin film transistors with high field effect mobility can be formed over an inexpensive glass substrate with a large area.
Some of the thin film transistors formed using the above semiconductor material would be unipolar transistors. Alternatively, even when both thin film transistors having p-type conductivity and thin film transistors having n-type conductivity can be formed, the number of manufacturing steps is remarkably increased because a p-channel thin film transistor and an n-channel thin film transistor are separately formed. Therefore, thin film transistors to be formed over the same substrate preferably have one conductivity. N-channel thin film transistors which have relatively high carrier mobility are particularly preferable.